2,4,5-Trichlorophenol is conventionally manufactured by high pressure, high temperature hydrolysis of 1,2,4,5-tetrachlorobenzene using methanolic sodium hydroxide. The overall tetrachlorobenzene hydrolysis is known to proceed via the intermediate 2,4,5-trichloroanisole. Two reactions occur simultaneously, viz. conversion of tetrachlorobenzene to trichloroanisole and demethylation of trichloroanisole to trichlorophenol. ##STR1##
It is also known that the impurity 2,3,7,8-tetrachlorodibenzo-.rho.-dioxin (TCDD) results from the condensation of two molecules of sodium trichlorophenate under conditions of high temperature and high alkalinity. ##STR2##
In tetrachlorobenzene hydrolysis, the concentrations of TCDD increase progressively with hydrolysis time. The rate of formation of TCDD is greater towards the end of the hydrolysis, being dependent on sodium trichlorophenate concentration. If the overall hydrolyses are run for progressively shorter periods, reduced concentrations of the desired 2,4,5-trichlorophenol are obtained.
It has now been found that, if the trichloroanisole demethylation is carried out at lower temperatures, under less alkaline or even acidic conditions, and for shorter reaction time periods that TCDD formation can be minimized.
It is known that some substituted anisoles may be demethylated by treatment with Lewis acids, although there are no literature references to the demethylation of chlorinated anisoles by this method.